1. Field of the Invention
The present invention relates to a paper conveying device for feeding paper by rotating rollers.
2. Description of the Related Art
An image forming apparatus, for instance, a copying machine takes out paper stacked in a paper supply cassette one sheet at a time with a pick-up roller by rotating it, supplies it by rotating a feed roller to aligning rollers provided in front of a photosensitive drum and keeps it waiting there. The paper being waiting is then fed to the photosensitive drum by the rotation of the aligning rollers timed with the rotation of the photosensitive drum.
The pick-up roller and the feed roller are kept contact with the surface of the paper and supply the paper by the friction of the contacting surfaces of the rollers. Therefore, the contacting surfaces of the rollers are gradually worn out with the increase in the number of sheets supplied and lastly, the paper supply capacity will drop. The excessively worn rollers must be changed with new rollers.
As a technique to detect the timing for exchanging the rollers, some methods are disclosed in the Japanese Patent Disclosures (Kokai) No. 2-37141(U), No. 3-56734(U) and No. 4-251042(P). These methods are to detect the exchanging times of rollers by measuring a length of time of paper to arrive a prescribed position after starting the paper supply.
However, in the case of a method to detect an exchanging time of a roller by measuring a length of paper conveying time as described above, for a temporary extended paper conveying time, which may result if extremely slippery paper is mixed, may be detected erroneously to be a roller exchange time.